Nutrient disinfectants
(Sterilizing agents)

Nutrient disinfection is essential for the prevention of diseases in
hydroponically grown plants.

Unlike soil grown plants, hydroponically
grown plants do not enjoy a protective root cover and the nutrient
solution is continuously exposed to the atmosphere. This allows the free
access of fungi such as pythium, phytophthora and fusarium (Fig 8.6).
Unfortunately,
these diseases spread readily when excess water is present and as a
preventative strategy, the nutrient solution needs to be regularly
disinfected.

There are a variety of disinfectants available for hydroponic systems.
“Oxidizing agents” have historically been most popular. Unlike alternative
methods their mode of attack ensures diseases will not develop resistance.
Oxidizing agents are also non systemic which means it does not enter the
plant. This enables disinfection treatment to continue throughout the
fruiting period without causing potential harm to the consumer. This
feature complies with ‘organic' farming guidelines that prohibit the use
of systemic chemicals.

Examples of oxidizing agents include chlorine dioxide, sodium
hypochlorite, monochloramine, hydrogen peroxide, UV and ozone. However,
their relative suitability for hydroponics is largely determined by their
“half-life” duration
and “oxidation potential”. These are the essential two properties that
determine whether or not the oxidizing agent can operate as a “broad
spectrum” disinfectant throughout the system. Note that a broad spectrum
disinfectant is required in hydroponics because of the variety of fungi
encountered. As shown below, monochloramine is most suitable.

‘Half-life’ duration: A long half-life means that when the
disinfectant is in the nutrient solution it remains chemically active for
a longer period of time. This ensures the entire system (root zone,
plumbing, etc) is treated each time the nutrient pump is turned on.
Without a sufficiently long half-life the disinfecting chemical becomes
inactive before it can reach the root zone. This is a deficiency with
ozone and UV. They only disinfect nutrient at the site of application.
Further, they can also destroy chelated trace elements
(e.g. iron EDTA, iron EDDHA).

Monochloramine has the longest half-life of all the oxidizing agents. This
is one of the reasons for its wide-spread use in disinfecting town water
supplies, especially where water is being pumped down long pipelines.
Although chlorine dioxide and sodium hypochlorite are widely used for
disinfecting town water supplies, their relatively short half-life
restricts their use to smaller town circuits where a shorter half-life is
tolerable.

Oxidation potential:
This is the measure of an oxidizing agent’s power or ability to kill fungi
and other organic organisms such as algae and slimes. Sufficient oxidation
potential also helps eradicate food sources that are responsible for
attracting fungus gnat and other root zone pests.
Although monochloramine, chlorine dioxide and sodium hypochlorite have
sufficient oxidation potential for the above mentioned tasks, only
monochloramine is ‘gentle’ enough for hydroponics. Chlorine dioxide and
sodium hypochlorite can cause harm to roots, 'organic' mediums and organic
growth promotants.

At the other end of the spectrum, peroxide is too weak for hydroponics. It
only becomes a strong oxidizing agent at pH’s below ~2, which is too low
for plant growth.